I agree with everything that Jasjit has written, i only wish to add that for the generations brought up in India where the caste system is still strong, that even though they don't discriminate generally (even thats come along way) it is sometimes difficlut for them to permit marriages into other castes. The reasons for this is cultural. One of the reasons why Sikhism came about was also to dispell myths and superstitions. The hindu's who still have a strong caste system and superstition system with no meaning, for example the story of Guru Nanak Ji throwing water in the opposite direction to the hindu pandits to water his field. The hindus were throwing water and food etc to reach the souls of their beloved dead ones. Guru Nanak was demonstrating the absurdity of it all.
Sadly certain rituals are still strong and vary from one caste sect to another, and sometimes it can be difficult for someone used to a different upbringing to settle into the routine of a family who have different rituals.
This is why sometimes when arranging marriages the parents have their childrens interests at heart and want them to settle. Those who are not born into a caste system are easier to accept sometimes because they do not have a series of rituals they perform, (for example not washing hair on a tuesday or thursday.. this is not sikh teaching)
Sikhism rightly doesn not beleive in the caste system, everyone is equal.